1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and the like, and more particularly, it relates to an image forming apparatus in which a sheet on which an image is formed is discharged onto a tray.
2. Related Background Art
In many image forming apparatuses, a sheet on which an image is recorded by a recording means is generally discharged onto a discharge tray. Such a discharge tray has also been used widely in ink jet image forming apparatuses which have recently been progressed. In such ink jet image forming apparatuses, since the recording is effected by discharging ink droplets onto a sheet, there is no problem when the normal recording (for example, having recording density for standard sentences and graphs) is effected. However, in case of high recording density such as recording density in color recording, an adequate ink fixing time period is required. In such ink jet image forming apparatuses, if an image forming speed is increased, a next sheet will be discharged before the ink is fixed to the previously discharged sheet, with the result that a tip end or a rear surface of the next sheet is contacted with the previous sheet, thereby distorting the image on the previous sheet.
To avoid such inconvenience, as disclosed in the Japanese Patent Laid-Open Application No. 1-235657, there has been proposed a method for promoting the drying of ink by using an infrared heater. When such a heater is used, the arrangement becomes complicated and the apparatus itself becomes more expensive. To avoid this, methods using no heater have been proposed.
For example, as shown in FIG. 21, a pair of opposed side rail members 50a, 50b are supported for pivotal movements around their pivot points 51a, 51b. Further, vertical walls 52a, 52b are provided with recesses 53a, 53b into which the side rail members 50a, 50b can be housed when the side rail members are rocked in directions indicated by arrows X. In this arrangement, a sheet S on which an image was formed is conveyed by a convey means (not shown) in a direction perpendicular to the plane of FIG. 21. In this condition, as shown in FIG. 21, the side rail members 50a, 50b are held in a closed position where the sheet S is supported by the side rail members to maintain the sheet S above a discharge portion 54a of a discharge tray 54.
When the sheet S is discharged, the side rail members 50a, 50b are rotated toward the directions indicated by arrows X to an open position where the sheet is not obstructed by the side rail members, thereby permitting the discharge of the sheet S onto the discharge portion 54a. In this way, the ink image recorded on the preceding sheet S discharged on the discharge portion 54a can be dried during the recording of the succeeding sheet, thereby preventing the contamination of the imaged surfaces of the sheets (refer to U.S. Pat. No. 4,794,859).
In another example wherein the heater is not used, there is provided a movable support arm for permitting the discharge of a sheet after the sheet reaches a terminal position of a convey path, and, while the sheet is being moved along the convey path, the arm is selectively positioned to contact with a central portion of a back surface of the sheet. As a result, during the recording, the succeeding sheet is maintained above a discharge tray portion and the ink image recorded on the preceding sheet discharged on the discharge tray can be dried, thereby preventing the contamination of the imaged surfaces of the sheets (refer to U.S. Pat. No. 4,844,633).
However, the above-mentioned conventional techniques line the following problems. In the former conventional technique, depending upon various sheet sizes, the side rail members 50a, 50b must be shifted in the sheet width-wise direction to meet with the selected sheet size. This requires a complicated mechanism. Further, when a large size sheet such as an A4 size (294 mm.times.420 mm) is used, the side rail members 50a, 50b must be enlarged in a sheet discharging direction, thereby making the apparatus bulky.
On the other hand, in the latter conventional technique, although the problems rising in the former conventional technique can be solved, the operation of the movable support arm must be synchronous with the sheet conveying operation. This requires a complicated mechanism.